Technology of integrating compound semiconductor elements on a silicon-on-insulator (SOI) substrate utilized as a communication optical device is attracting more attention. Particularly, technology of integrating a laser and an optical modulator is important as to dramatically increase integration density of communication devices and to reduce the cost. For an optical modulator that needs higher speed and efficiency, a charge-accumulation-type optical modulator with a MOS structure including an n-type InP layer and a p-type Si layer with little optical loss has potential to achieve those needs (see non-patent literature 1).
As shown in FIG. 4, this optical modulator is formed from a p-type silicon layer 402 formed on a buried insulating layer 401, an insulating layer 403 formed on the p-type silicon layer 402, and an n-type InP layer 404 formed on the insulating layer 403. A p-electrode 405 is connected to the p-type silicon layer 402, and an n-electrode 406 is connected to the n-type InP layer 404. The buried insulating layer 401 functions as a clad. The p-type silicon layer 402 is formed by patterning an SOI layer 411 on the buried insulating layer 401. An air groove 412 is formed in a side portion of the p-type silicon layer 402 on the formation side of the n-electrode 406 with the SOI layer 411. The upper surface of the n-type InP layer 404, which is not covered with the n-electrode 406, is open to a space.
Part of the p-type silicon layer 402 and part of the n-type InP layer 404 are arranged so as to overlap each other in a planar view, and form an optical waveguide portion where light to be modulated is guided. In this optical waveguide portion, the insulating layer 403 is sandwiched between the p-type silicon layer 402 and the n-type InP layer 404 in a stacking direction. Light to be modulated is confined in a light confinement portion including clad regions consisting of buried insulating layer 401, the air groove 412, an upper space, and the like, thereby forming a waveguide mode 421.
In this optical modulator, by modulating the concentration of majority carriers accumulated at the interfaces between the p-type silicon layer 402 and the n-type InP layer 404 with the insulating layer 403 in the optical waveguide portion, the refractive indices of the p-type silicon layer 402 and the n-type InP layer 404 in the optical waveguide portion are modulated. In this structure, an improvement in modulation efficiency is expected to be about two to three times greater, as compared to a case in which Si is used for an n-type layer.